How to prevent noise from power adapter

　　Noise on the power adapter: If it is a linear power adapter, first of all, the low frequency 50Hz is a serious interference source.

　　Since the AC power coming in from the primary is not pure, and it is a sine wave of waves, it is easy to cause electromagnetic interference, that is, electromagnetic noise, to the circuit next to it. If it is a switching power adapter, the noise is more serious. The switching power adapter works in a high-frequency state, and there is a very dirty harmonic voltage in the output part, which can generate a lot of noise to the entire circuit.

　　Prevention methods: reasonably ground, use differential structure to transmit analog signals, add decoupling capacitors to the output of the power adapter of the circuit, use electromagnetic shielding technology, separate analog and digital grounds, take the bottom line on both sides of the signal line, and isolate the ground line. In fact, what I said about noise removal is only the tip of the iceberg. Even people who have been playing electronics for 30 years will not fully master all of these technologies, because understanding and mastering such things requires a strong technical foundation and a wealth of knowledge. Experience, but what I told you is generally enough. The background noise is caused by the circuit itself, due to the impurity of the power adapter, improper phase margin and gain margin of the circuit, etc., the circuit itself and the device. This part needs to be improved during circuit design. Other noises are due to unreasonable circuit layout and other factors, such as electromagnetic compatibility, interference between wires, and so on.

　　The elimination of noise in analog circuits relies more on experience rather than scientific evidence. The situation that designers often encounter is that after the analog hardware part of the circuit is designed, they find that the noise in the circuit is too large, and they have to redesign and route. This "try and see" design method can finally be successful after several twists and turns. However, a better way to avoid noise problems is to follow some basic design guidelines when making decisions in the early stages of design, and use knowledge of basic principles related to noise.

　　Design method of low-noise preamplifier circuit

　　The preamplifier plays an important role in the audio system. This article first explains how engineers should select components appropriately when designing preamplifiers for home audio systems or PDAs. Subsequently, the source of noise was analyzed in detail, which provided guidelines for the design of low-noise preamplifiers. Finally, taking the preamplifier of a PDA microphone as an example, the design steps and related precautions are listed.

　　Pre-amplifier refers to the circuit or electronic equipment placed between the signal source and the amplifier stage, such as an audio pre-amplifier placed between the CD player and the power amplifier of the advanced audio system. The pre-amplifier is designed to receive weak voltage signals from the source. The received signal is amplified with a small gain, and sometimes even adjusted or corrected before being transmitted to the power amplifier stage, such as audio pre-amplifier The amplifier can first equalize the signal and perform tone control. Whether designing a preamplifier for a home audio system or a PDA, you have to face a very headache, that is, which components should be used?

　　Component selection principle

　　Due to the compact size and excellent performance of operational amplifier integrated circuits, many preamplifiers currently use this type of operational amplifier chip. When we design a pre-amplifier circuit for a sound system, we must clearly know how to select the appropriate technical specifications for the operational amplifier. In the process of power adapter design, system design engineers often face the following problems.

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